Backward flow travelling wave oscillators



March 31, 1959 psz 2,880,355

BACKWARD FLOW TRAVELLING WAVE OSCILLATORS Original Filed April 9, 1952 m EEEEEQEEEQ INVENTQR BERNARD EPSZTEIN BACKWARD FLOW TRAVELLING WAVE OSCILLATORS Bernard Epsztein, Paris, France, assignor to 'Compagnie l(Eenerale de Telegraphic Sans Fil, a corporation of rance Original application April 9, 1952, Serial No. 281,347.

Divided and this application March 11, 1958, Serial No. 720,663

Claims priority, application France April 13, 1951 19 Claims. (Cl- 315-3-5) I tial energy, but their efiiciency is not high and their tuning band is extremely narrow. It is only by employing mechanical tuning devices providing changes in the volume of cavity resonators that frequency variations of some 25% may be obtained. Magnetrons have a good efficiency, but the tuning possibilities are poor and involve substantial complications. Both for klystrons and for magnetrons, good matching of the load is necessary for good efliciency.

It is an object of the present invention to provide an ultra high frequency oscillator which avoids the disadvantages of the above devices, while combining their known advantages. In other words, the oscillator according to the present invention is designed preferably to pro- ""vide substantial energy with good efficiency and to be tunable continuously without mechanical complications and throughout a wide frequency band.

The oscillator according to the invention belongs to the class of tubes known in present day practice as travelling wave tubes. Such tubes comprise essentially an electron y source capable of emitting an electron beam, which is caused to propagate from this source to a target, and a delay line in coupled relationship with the beam and along which a wave of high frequency energy is caused to propagate in the same direction as the beam. If the apparent or phase velocity of one of the components of this Wave propagating from the source end of the delay line toward the target end thereof is substantially equal to and in the same direction as the velocity of the beam, interaction occurs between the beam and this wave, and the-latter is amplified.

Some attempts have been made'to operate these travel- 1 ling wave tubes as oscillators, but they do not have a: continuous tuning band throughout the range of frequencies for which the tubes are "designed. Hence, they have been used mainly only as amplifiers.

According to the present invention, a tube of the travelling wave type is caused to operate as an oscillator,-

tunable throughout a wide, uninterrupted frequency band.

' For this purpose, the tube is provided with a delay line which has a geometrically periodical structure, i.e., the

shape of which is periodically repeated in space at regular "intervals, and--which is electrically aperiodical" for the frequency band within which the tube is designed to oscillate. Generally speaking, any delay line of geometrica'lly-periodical structure'may be used which is capable of sustaining only travelling waves at least ina given band. electrical aperiodicity .may. be obtained by United States Patent 2,880,355 ra ented Mar. 31, 1959 substantially matching or adapting the line to suppress reflections of energy from the end of the line remote from the source, at least in the selected band, and in practice the line is provided, at least at the end thereof remote from the source of electrons, with means for absorbing the high frequency energy propagating in the same direction as the beam and which would otherwise be reflected from that end, while at the end adjacent to the source of electrons means are provided for collecting high frequency energy propagating backwards in the opposite direction to the beam and for transmitting it to a load or utilization circuit.

The operation of the tube according to the invention is-based upon the interaction which will occur between the beam and the wave energy propagating in the delay line in the backward direction, i.e., in the opposite-direction to the beam, when the apparent or phase velocity of one of the components of this wave is substantially equal to and in the same direction as the velocity of the beam.

The invention will now be particularly described with reference to the appended drawings showing some embodiments thereof and wherein:

Figs. 1 and 4 are longitudinal sections of two tubes according to the invention; and

Figs. 2 and 3 show two embodiments of delay lines among those which may be utilized in tubes according to the invention.

Referring now to the embodiment shown in Fig. 1, it may be seen that the oscillator according to the invention comprises essentially a tube ofthe travelling wave type, which may be for instance of rectilinear form. This tube comprises, within an evacuated envelope 7, electron emitting'means such as a conventional electron gun 2 and a delay line 3 which may be of interdigital, zig-zag, helicoidal or other structure or which may comprise vanes or discs supported by a rod. The electron gun 2, the cathode of which is at a negative potential with respect to the line 3, provides, in a manner well known in the art, an electron beam 1 which propagates towards a target or collector 6 to which a positive potential V with respect to the electron source is applied in a conventional manner.

According to a preferred embodiment shown in Fig. 1, electron optical means are provided in the form of crossed electrical and magnetic fields, perpendicular to each other and to the direction of travel of the beam, for directing and controlling the' electron beam-in-the oscillator according to the invention. As described for "instance in U.S. Patent No. 2,511,407, such an arrangement improves the operation of the beam in travelling wave tubes. While these fields are provided in the embodiment according to Figure 1, they are not provided in the embodiment according to Fig. 4. In the embodiment shown in Fig. 1, the lines of force of the magnetic field are perpendicular to the sheet of drawings and are shown at 5 in a conventional-way. The electricfield,

: having its lines of force perpendicular to the beam and the delay line 3 to define therebetween an interaction wave travelling in connection 9. The delay line 3 is provided with attenuating means forabsorbing any energy reflected by the load circuit 8. Such attenuating means may be in- I herent ,to the delay line 3, but in the embodiment shown lcollector 6. grid 13. Further, conventional electron optical-means 'areprovided which comprise an.acceleratinganode15 and afocusing electrode 14.

in-Fig. 1, it isin the form of an absorbing layer on the surface of a portion 10 or the line 3 at the end thereof remote from the load although it is also possible to match ,that end of the line. by. anyv other. known, internal or 1 external attenuatingor absorbing means.

To cause the. above described oscillator. of Fig. 1 to oscillate, applicant proceeds in the following manner: Electron gun 2 is made to operate as also the source V and the magnetic field 5, and beam 1 propagates from the gun 2 towards the target 6. This beam interacts with the high frequency energy which appears at the same .moment in the delay line a. .l-Iow .this energy appears will be described later. high frequency waves propagate in that direction which,

As a result of. this interaction,

for the sake of simplicity, will be termed the positive. di-

-The tuning of theoscillator is obtained by-changing the propagation velocity of the beam. This may for instance be obtained in the embodiment shown. in Fig. l more generally by varying the ratioof the electric field and the magnetic field-or more specifically by varying =the potential between the electrode or plate 4 and the line 3 by adjusting the potentiometer 12. By so proceeding, the frequency of the oscillator may be uninterruptedly varied at will within a bandwidth as large as 30 to 50% of the lowest frequency for which the tube is designed to operate, while according to present day practice oscillators capable of providing, without mechanical tuning, a bandwidth up to 1% of the lowest frequency were considered as very satisfactory.

Any means known in the art may be utilized to modulate the output of the oscillator. Amplitude modulation may be performed by amplitude modulating the beam 1, for instance by means of a grid provided in the path of .this beam. Frequency modulation may be performed by applying a modulation voltage to the circuit provid- -.ing the electric field as shown at 11 and as is well known in. the art.

The invention is by no means limited to the shape I of tube shown in Fig. 1. For example, the tube may be of circular form as described in U.. S. Patent No.

In. the" tubeof Fig. 4, vno means are provided.v for creating. electrical ormagnetic fields perpendicularv to ""eachother and to. the beam. This tube comprises an ztemissive cathode 2, which provides the. beam 1,.adelay .-.line 31having for example. an interdigital structure and .an attenuator portion which may be in any of the forms suggested in the case of Fig. 1,. and a target or The electron-density is controlled by a An output antenna it is coupled to the end of the delay line adjacent to the gun 2 andmatching means 16 may be provided to matchthe antenna to theline.

If desired, a longitudinal magnetic field may be provided by conventional means.

Oscillation of the tube of Fig. 4, is achieved in the same way asin the case of Fig. l, and variationofthe frequency of oscillation is obtained similarly by adjust- .ing a potentiometer 12, not shown in Fig. 4.

While applicant believes that the following provides a It is known that ifthe delay line'3 is of geometrically periodical structure and iselectrically aperiodicab within the operating band of the tube, a flow of energy of a given frequency propagated along the line in a given direction may be considered as being formed by the superimposition of an infinity of progressive waves all having the same frequencyotbut each having a different apparent or phase velocity and some being directed forwards and others backwards along the line. These waves are usually called space harmonics, some of which are termed positive because they are directedin the same direction as'the flow of energy and the others of which are termed negative because they are directed inthe opposite direction to the flow of energy in the line.

Furthermore, it.is known that when a high frequency wave is propagating along the delay line. of a conventionaltravelling wave tube and an electron beam is being emitted by tl'iegun' 2 in 'the' directionof the' target 6, interaction occurs betweenthe beam. and that space harmonic the apparent, delay or phase velocity of which is substantially equal .tothe-velocityofthe electronscf the beam. land which is directed in the same. direction. It

isknown thatthe desired velocity v ofLthe electrons may be obtained in the form of tube of Fig. l by adjusting 1 either or both of the parameters represented by the electrio and magnetic fields to appropriate values E -in volts per meter and B in webers per square meter, since B v being in meters per second. In" the form of tube of Fig. 4,.the velocity v of the electrons isdetermined by the formula tive space harmonic. of high frequency electro-magnetic wave energy induced in the line by the noise inherently accompanying the electron beam, this space harmonic directed in the'same direction. as the beam,.i.e.,-in.the

direction which may be termed the negativedirection with respect to the how of energy, interacts with the beam,- and amplified energyis causcdto flow in the positive or backwarddirection. Thisresults in .an increased energy flow towards the gun ofthe delay line-.011 the frequency for which interactlon isobtained. (This increased. flow of energy: further 'modulates .the.beam,

giving rise to: a further increased flowof energyaand'so on. .If-..the beam currentislarge. enoughthis internal feedback. mechanism results inself-sustained oscillations on a frequencysubstantially determined by the velocity of the electrons for a delay line of given structure.

.; Figs. 2' and. 3 show. delay lines which areespecially suitable for" usein tu'bes'accordingtothe invention.-.- The delay line of Fig. 2 is a symmetrical interdigitalline and the: delay line'of Fig. 3.is a zig-zag line.

if the matching between the load and the line is per 1 feet, a. maximum transfer of energy. is .efiected. from1.the

.tube.to the load. Even if, by reason of somemismatching, some reflection'occurgthis takes place in .the. negative direction. and absorptionoccurs at theend 10 so that there is no interaction thereof-withthe beam and there :is. no frequency pulling.

' :It should beespeciallynoted that no couplingztakes :place betweenthe ends of the. delay line contrary towhat takes .place in. the magnetron.

1 Thus it 'has been'shown that the invention provides'an uninterrupted-wide 'band ultra' high frequencyioscillator operating onan internal feedback principle and-the .opera- .tion' of which is not substantially affected insofar as -fre .fquency stability is concerned byithe existence of some 1 mismatching betweemtheload and the-delay line.

I claim: 1

1. An ultra high frequency oscillator tube adapted to produce oscillations of a frequency which is adjustable at will over a relatively wide, uninterrupted band, said tube comprising: a delay line having a geometrically periodical structure, an electron emissive source adjacent one end of said line and positioned to emit a beam of electrons in coupled relationship with said line thereby to induce electromagnetic wave energy in said line and to interact with a space harmonic thereof, said line having its two ends mutually uncoupled, means disposed at least at the other end of said line for absorbing ultra high frequency energy propagated along said line in the same direction as the beam thereby substantially to prevent reflections from said other end and thus to render said line electrically aperiodic within the limits of said band, means for directing said beam of electrons along a path substantially parallel to said line and at a velocity substantially equal to the apparent or phase velocity of a negative space harmonic of electromagnetic wave energy propagating in said line in the opposite direction to the beam, thereby to cause interaction between said beam and said negative space harmonic for sustained flow of energy toward said electron emissive source, and means adjacent said source for transferring said energy to an external load circuit.

2. An oscillator tube as in claim 1 further comprising means for varying the velocity of the electrons of the beam thereby to vary the frequency of oscillation of the tube.

3. An oscillator tube as in claim 1 further comprising means for applying a difference of potential between said line and said source and means for varying said difference of potential thereby to vary the frequency of oscillation of the tube.

4. An oscillator tube as in claim 1 wherein said absorbing means are positioned at said other end of said line.

5. An oscillator tube as in claim 1 wherein said absorbing means comprise an absorbing layer on a portion of said line at said other end thereof.

6. An oscillator tube as in claim 1 wherein said line is of symmetrical interdigital structure.

7. An oscillator tube as in claim 1 wherein said line is of zig-zag structure.

8. An oscillator tube as in claim 1 wherein the means for directing said beam of electrons emitted by said source along said path and at said velocity comprise an electrode extending parallel to said line, power terminal means at said line and said electrode for establishing thereber tween an electrical field having lines of force extending in a direction perpendicular to said path of the electrons, and means for establishing a magnetic field having lines of force extending in a direction perpendicular to the lines of force of said electrical field and to said path.

9. An oscillator tube as in claim 8, further comprising means for varying at will the ratio between the values of said electrical and magnetic fields thereby to determine the frequency of oscillation of the tube.

10. An oscillator tube as in claim 8 further comprising means for applying alternating current to a circuit including said line and said electrode thereby to effect frequency modulation.

11. An oscillator tube as in claim 1 wherein the means for directing said beam of electrons along said path and at said velocity comprise a focusing electrode and an accelerating electrode associated with said source.

12. An ultra high frequency tube, said tube comprising a delay line having a geometrically periodical structure, an electron emissive source adjacent one end of said line and positioned to emit a beam of electrons in coupled relationship with said line thereby to induce electromagnetic wave energy in said line and to interact with a space harmonic thereof, said line having its two ends mutually uncoupled, means disposed at least at the other end of said line for absorbing ultra high frequency energy propagated along said line in the same direction as the beam thereby substantially to prevent reflections from said other transferring said energy to an external load circuit.

.13. An ultra-high-frequency oscillator tube adapted to produce oscillations of a frequency which is adjustable at will over a relatively wide band, said tube comprising a-delay-line, an electron emissive source adjacent one end of said line and positioned to emit a beam of electrons in coupled relationship with said line thereby to induce electromagnetic wave energy in said line and to interact with a space harmonic thereof, said line having its two endsmutually uncoupled, means for directing said beam of electrons along a path substantially parallel to said line and at a velocity substantially equal to the apparent or phase velocity of a negative space harmonic of electromagnetic wave energy propagating in said line in the opposite direction to the beam, thereby to cause interac' tion between said beam and said negative space harmonic for sustained flow of energy toward said electron emissive source, and means adjacent said source for transferring said energy to an external load circuit.

14. An ultra-high-frequency oscillator tube adapted to produce oscillations of a frequency which is adjustable at will over a relatively wide, uninterrupted band, said tube comprising a delay line, an electron emissive source adjacent one end of said line and positioned to emit a beam of electrons in coupled relationship with said line thereby to induce electromagnetic wave energy in said line and to interact with a space harmonic thereof, said line having its two ends mutually uncoupled, means operatively connected with said line to minimizerefiections of energy from the end of said line remote from said electron emissive source, means for directing said beam of electrons along a path substantially parallel to said line and at a velocity substantially equal to the apparent or phase velocity of a negative space harmonic of electromagnetic wave energy propagating in said line in the opposite direction to the beam, thereby to cause interaction between said beam and said negative space harmonic for sustained fiow of energy toward said electron emissive source, and means adjacent said source for transferring said energy to an external load circuit.

15. An ultra-high-frequency oscillator tube according to claim 14, wherein said means for minimizing reflec- ;ions is constituted by attenuating means inherent in said 16. An ultra-high-frequency oscilaltor tube according to claim 14, wherein said means for minimizing reflections includes external attenuating means operatively connected to said delay line.

17. An ultra-high-frequency oscillator tube adapted to produce oscillations of a frequency which is adjustable at will over a relatively wide band, said tube comprising a wave-guiding structure having effectively a first end and a second end, cathode means for emitting electrons adapted to propagate from said first end toward said second end along said wave-guiding structure in coupled relationship therewith to thereby induce electromagnetic wave energy in said wave-guiding structure and to interact with a space harmonic thereof, means for directing said electrons to propagate along said wave-guiding structure in coupled relationship therewith in a direction from said first end toward said second end of said wave-guiding structure and at a velocity substantially equal to the apparent or phase velocity; of anegative spaceiharmonic ofthe-felectromagnetic Wave'energy propagating in said-wave-guiding structure in the opposite directionto the flowbof said electrons, thereby to cause interaction between said electrons and said negative space harmonic for sustained flow .of energy toward said cathode means, and;means, near said first end of said wave-guiding structure and 'zoperatively connected thereto for transferring said energy to an external load circuit.

18, An ultrahigh-frequency oscillator tube adapted to produce oscillations of a frequency which is adjustable at will over a relatively wide band saidltube comprising a wave-guiding structure having effectively a first .endgand a second end, cathode means for emitting -,elect1:ons

said first end toward said second end of said wave-guiding structure and at a velocity substantially equal to the apparent or phase velocity of a negative space harmonic of the electromagnetic wave energy propagating in said waveguiding structure in the opposite direction to the flow of said electrons, thereby to..cause interaction between said electrons and said negative space harmonic for sustained flow of energy toward said cathode means, means near said first end of said wave-guiding structure and operatively connected thereto for transferring said energy to an external load circuit, and means at least near one end of said wave-guiding structure to minimize reflections r: thereat. of :said induced electromagnetic wave.

. 19. A v,travelling-wave, ultrahigh-frequency. electron discharge deviceadaptedqto produce oscillations of a .:frequency which isradjustableat will over a relatively wide band,=said-tube comprising a wave-guiding structure ,having efiectively a'firstgend and a second end, cathode 1 means in :said vsystem for emitting electrons adapted to propagate from said first end toward said second end along ,saidwave-guidingstructure in coupled relationship therewith to thereby induce electromagnetic wave energy in said wave-guiding structure and to interact with a space ,harrnonic, thereof, means forv directing .saidt electrons to propagate alo ng;said wave-guidingstructure in coupled towardsaid secondhendioigsaid ,wave-guiding structure relationship therewithin a directionfrorn said first; end

and at a'velocity substantially equalto.the apparent- .or :phase :velocity of a negative spaceharmonic of the electromagnetic wave energy propagating in said wave-guiding structuretin :the oppositehdirection to the flow of said electrons, thereby to cause interactionbetween said electrons and said negative space harmonic for sustained flow of energy'toward said cathode means, and means near said first end of said wave-guiding structure and operatively connected thereto for transferring said energy to an external load circuit.

References (Iited in the file of this, patent UNITED- STATES PATENTS 2,699,519 Bruck Jan. 11, 1955 2,794,143 *Warnecke et al. -May 28, 1957 2.8 16 Lindenblad Aug. 4, 1957 

